The Research On Ring-Opening Polymerization Of L-Lactide And ε-Caprolactone Initiated By Rare Earth Schiff-base Complexes

During recent decades biodegradable aliphatic polyesters have been attracting considerable attention from researchers due to their potential application in the biomedical,pharmaceutical,and agricultural fields.Poly(L-1 actic acid)(PLLA) and poly(ε-caprolactone)(PCL) are two of the more important polymers among biodegradable aliphatic polyesters.PLLA,PCL and their copolymers are widely used as surgical sutures,controlled drug delivery,bone fixation device,tissue repair materials and other biomedical stuff due to their special advantage on degradability rate,melt temperature,drug permeability and biocompatibility.Hitherto,several organometallic compounds mainly involving stannous, aluminum,calcium,zinc,magnesium and iron,have been successfully used to initiate the ring-opening polymerization of L-lactide(L-LA) andε-caprolactone(ε-CL).During recent years,Rare earth initiators have received much current interest due to their low toxicity and high activity.Schiff base complexes are easy to synthesize and handle.It was able to form stable complex with transition metals and rare earth element which attributed to its special structures.In this paper,rare earth tris(Nphenylethyl -3,5-di-t-butylsalicylaldiminato)s[Ln(OPEBS)3]complex is firstly used as the ring-opening polymerization of L-LA andε-CL and their copolymerization,and achieves good results.[Ln(OPEBS)₃]is a new and effective initiator for ring-opening polymerization of L-LA.The initiating activities of different rare earth complexes[Ln(OPEBS)₃]is as follows:La＞Sm＞Y＞Dy.La(OPEBS)₃ has the highest activity among the four initiators under the same polymerization conditions.The influences of solvents,the molar ratio of the monomer to initiator(M/I),monomer concentration,reaction temperature and time on the ring-opening of L-LA are investigated systematically using La(OPEBS)₃ as the initiator.The result shows that the best polymerization condition for the ring-opening of L-LA is as follows:toluene as the solvent,[L-LA]= 2.0 mol/L,[L-LA]/[La]= 200(molar ratio),temperature= 80℃,time= 5 min Under this experiment condition,PLLA with a yield of 91.2%,number-average molecular weight of 1.60×10⁴ and polydispersity of 1.53 can be obtained.The structure and characteristic of PLLA are characterized by GPC,DSC and ¹H NMR in detail. Mechanism investigation shows that L-LA proceeds through a "coordination-insertion" mechanism with selective rupture of acyl-oxygen bond.It has been found that La(OPEBS)₃ is an effective initiator forε-CL polymerization.The effects of solvents,the molar ratio of the monomer to initiator (M/I),monomer concentration as well as temperature and polymerization time were examined systematically using La(OPEBS)₃ as the initiator.PCL with M_n around 5.84×10⁴ and polydispersity of 1.57 in high conversion(99.9%)under following polymerization conditions:[CL]=2.0mol/L,[CL]/[La]=1000(molar ratio),40℃,40 min,toluene as the solvent.The mechanism ofε-caprolactone polymerization by La(OPEBS)₃ can be described as a coordination insertion,acryl-oxygen cleavage.In order to obtain more practical biodegradable materials,the copolymerization of L-LA andε-CL using La(OPEBS)₃ as the initiator was explored.The influences of monomer added order,monomer feeding ratio,the molar ratio of the monomer to initiator(M/I) and the polymerization temperature on the copolymerization of L-LA andε-CL have been studied.The GPC and ¹H NMR analysis of the product extracted by petroleum ether confirms that the product is the block copolymer.